BOOL isRunning = NO;
do {
isRunning = [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDatedistantFuture]];
} while (isRunning);
dispatch_sync 和 dispatch_async 区别：
dispatch_async(queue,block) async 异步队列，dispatch_async 函数会立即返回, block会在后台异步执行。
dispatch_sync(queue,block) sync 同步队列，dispatch_sync 函数不会立即返回，及阻塞当前线程,等待 block同步执行完成。
- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"=================4");
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"=================5");
});
NSLog(@"=================6");
}
可能会出现线程死锁。